In 1972, the National Institue of Health in collaboration with the Immunology Branch of the National Cancer Institute initiated a breeding program to develop a highly inbred strain of miniature swine. From its inception, this herd has traditionally produced an average litter size of approximately five offspring per animal, considerably less than the mean litter size of eight-12 reported for standard sized domestic pigs. Unlike their standard sized counterparts little information on behavioral-ovarian-endocrine relationships exist for the miniature pig. The present study characterized these reproductive traits and determined that gross ovarian morphology and duration of estrus and the estrous cycle were similar between the miniature pig and previous reports for standard sized animals. However the miniature pig produced fewer ovarian follicles and had a reduced ovulation rate and depressed serum concentrations of hormones when compared to standard sized animals. The data suggest that the reduced litter size detected in this particular strain of miniature pig was, at least in part, the result of a reduced ovulation rate.